1. Field of the Invention
The present invention is directed to an improvement of an auxiliary valve mechanism in a slide type exhaust brake system, connectively interposed in an exhaust pipe generally in a large-sized vehicle such as a truck, a bus or the like, for imparting braking effects to the vehicle by blockading an exhaust passageway of an exhaust gas flowing through the exhuast pipe when travelling on sharp slopes.
2. Description of the Prior Art
One typical arrangement of a prior art auxiliary valve mechanism nit of exhaust brake system of this type will now be described. As illustrated in FIGS. 4 and 5 (see U.S. patent application Ser. No. 146,841 filed on Jan. 22, 1988, now U.S. Pat. No. 4,819,696). Fitted to the top of a piston rod (26) is an auxiliary valve mechanism consisting of: a support frame (28) mounted on the front surface of a block member (25) engaging with a valve member (23) provided in the vicinity of the top of piston rod (26), the support frame accommodating a tabular liner (29), both lengthwise ends of which are bent upwards to assume a sunk configuration, and sliding on the bottom wall surface of valve member (23). A single piece of exhaust pressure adjusting hole (24) penetrates the surface of valve member (23) incorporated in a reciprocatory passageway provided within a housing of a body. When opening and closing the exhaust passageway, the top of piston rod (26) and the side wall of block member (25) engage with or impact on valve member (23), whereby the valve member is pulled up while being thrust forward and moved backward.
Note that the numeral (30) denotes a small diameter exhaust pressure escape hole, formed in valve member (23), for preventing an excessive load on the machinery in a state where the exhaust passageway is closed.
Based on the prior art auxiliary valve mechanism, however, the following problems will arise. The sunk state of liner (29) loosely fitted to the interior of support frame (28) causes to-and-fro or right-and-left sway as the liner slides on the surface of valve member (23) when opening and closing the exhaust passageway, resulting in the uncertainty of tight fitness. Part of exhaust gas is discharged from escape hole (30) in the case of closing the exhaust passageway, depending on the posture that the exhaust brake system takes when being disposed. In addition, there is created a gap in the sliding surfaces between the liner and closed exhaust pressure adjusting hole (24). Then, the exhaust gas escapes from this gap, which tends to cause an incipient delay in braking and also a loss of braking effects.
Furthermore, the single piece of exhaust pressure adjusting hole (24) does not have a large diameter but has a limit to its dimension in connection with a size of valve member (23) in a confined space. Hence, a sufficient amount of drop in exhaust pressure can not be obtained when opening exhaust pressure adjusting hole (24) in advance of opening the exhaust passageway, thereby exhibiting poor respondency at the valve opening time. A slide range of auxiliary valve mechanism (27) on the surface of valve member (23) tends to increase, with the result that a comparatively lengthy air cylinder unit incorporating a relatively strong spring has to be employed. This brings about an increment in weight of the whole brake system. In some cases there is produced an obstacle to a condition under which the brake system is installed in the confined space.